Chapter 19 – Reaction Rates and Equilibrium

Rates of ReactionReversible Reactions and EquilibriumDetermining Whether a Reaction Will OccurCalculating Entropy and Free EnergyThe Progress of Chemical Reactions

19.1 Rates of Reaction

How long does a chemical reaction take?

Collision Theory IF colliding particles

have enough energy (KE) and collide at the right orientation, they react to form a new product:

19.1 Rates of Reaction (cont.)

The minimum amount of energy that particles must have in order to react is called the activation energy.

Activation energy acts like a barrier that the reactants must cross to be converted into products.

Activated complex, transition state…

19.1 Rates of Reaction (cont.)

Factors Affecting Reaction Rates Here they are

again… Temperature Concentration Particle size Catalysts

19.2 Reversible Reactions and Equilibrium

Reversible Reactions Reactions that can occur in the other direction.. 2SO2 + O2 2SO3

19.2 Reversible Reactions and Equilibrium (cont.)

Chemical Equilibrium A state in which the forward and reversible reactions

take place at the same rate.

Factors Affecting Equilibrium: Le Chatelier’s Principle Obviously, when discussing equilibrium there is “a fine

line” of balance in the reaction – changes of almost any kind can disrupt this balance.

Le Chatelier’s Principle: If a stress (temp, conc., pressure) is applied to a system in dynamic equilibrium, the system changes to relieve the stress.

19.2 Reversible Reactions and Equilibrium (cont.)

Equilibrium Constants A ratio of product concentrations… aA + bB cC + dD

Keq = [C]c x [D]d / [A]a x [B]b

where [X] is expressed in mol/L and x is the number of moles from the balanced chemical equation.

A Keq value > 1 favors products at eq.

Keq value < 1 favors reactants at eq.

Examples…

Ch 19 Assignments

Ch 19 CPQs #1 pg. 572 #38,39,40,41,43,44,46,47,48,49,50

19.3 Determining Whether a Reaction Will Occur

Free Energy and Spontaneous Reaction To do work, you need available energy. Free energy: energy that is available to do work. (Not

always efficient. Reactions may OR may not occur spontaneously…

Entropy Reactions tend to occur to attain the lowest possible

energy for a system: the law of disorder. The disorder in a system is measured as entropy, (S).

19.3 Determining Whether a Reaction Will Occur (cont.)

Entropy explained and visualized…

19.3 Determining Whether a Reaction Will Occur (cont.)

Entropy explained and visualized…

19.3 Determining Whether a Reaction Will Occur (cont.)

Entropy explained and visualized…

19.3 Determining Whether a Reaction Will Occur (cont.)

Entropy explained and visualized…

19.3 Determining Whether a Reaction Will Occur (cont.)

Heat, Entropy, and Free EnergyThe size and direction of heat

(enthalpy) changes combined with entropy determine whether or not a reaction is spontaneous, i.e., whether it favors products and releases free energy.

Table 19.1, pg. 554

19.3 Determining Whether a Reaction Will Occur (cont.)

19.4 Calculating Entropy and Free Energy

Entropy Calculations Standard entropy change (entropy change at standard

conditions: So = So (products) – So (reactants) Use Table 19.2 Examples…

Free Energy Calculations The maximum amount of energy that can be combined with

another process to do work: Gibbs Free-Energy (G). G = H – TS where H = enthalpy, T = Temperature Go = Go

f (products) – Gof (reactants)

Tables 19.3 and 19.4 Examples…

19.5 The Progress of Chemical Reactions

Rate Laws Fortunately, we don’t have to settle for explanations that

are qualitative in nature- in other words, “the reaction will likely favor the products…”

Mathematically calculating rates of reaction… Rate Law: an expression relating the rate of a reaction

to the concentration of reactants: Rate = A / t = k x [A]

For the reaction aA + bB cC + dD, the rate equation takes the form:

Rate = k[A]a[B]b

19.5 The Progress of Chemical Reactions (cont.)

Rate Laws (cont.) The specific rate constant,k, determines the relationship

between the concentrations and the rate of reaction. If k is large, the reaction will form products quickly. If k is small, the reaction will form products slowly. The order of a reaction is the power to which the

concentration must be raised to give the experimentally observed relationship between concentration and rate: 1st order reaction – proportional to one reactant.

Reaction mechanisms… Showing the reactions involved in a complex reaction Figure 19.3, decomposition of N2O…

Ch 19 Assignments

Ch 19 CPQs #1 pg. 572 #38,39,40,41,43,44,46,47,48,49,50

Ch 19 CPQs #2 pg. 572 #51,53,54,55,56,59,61,62